Double Gang Switch Boxes and Switches

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  #1  
Old 05-30-01, 10:24 AM
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Question

I need some advice from both a NEC point of view and a safety point of view (which, hopefully, are one and the same).

I am planning the wiring for my basement finish project. Like many basements, I have a support pole (just less than 4" in diameter) in the middle of my basement holding up the middle of my house. I would like to box that pole in with a 12 inch by 12 inch square. Doing so would give me a 13" face (once you add the drywall depth) on each side of the square column.

Here's where the electrical part comes in. I would like to install a double gang switch box on each side of the square -- all at the standard height for switches. So, I would have eight switches spread over four double gang boxes in this one square foot area (with the support pole in the middle of it). All of this will fit in the area with plenty of working room to spare.

My concern is the back to back placement with these double gang boxes. From the back of one box to the back of another would be about six inches (although there will be the metal pole in between them).

I am using fiberglass boxes for my double gang boxes (I'm not sure if I can mention the brand name or not).

Any comments on this setup would be most appreciated. If I can't put all of the double gang boxes at the same height, can I still do this setup if I put one box at 36", one at 42", one at 48", and one at 54"?

I should state that I will have other switches by the entry door into the room and such but it would be real convenient if I could put these eight switches at this central location and more attractive if I could spread them out over the four faces of this central column.

Thanks. -- Robert
 
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  #2  
Old 05-30-01, 11:26 AM
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Robert, I'm unclear as to what potential problem you might be worried about. If you're concerned about heat buildup, then I don't think this will be even remotely close to a problem. And I can't think of any other potential problem you might be anticipating. Go for it!
 
  #3  
Old 05-30-01, 11:36 AM
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John,

Thanks for the comments. Heat buildup was one of my main concerns although I, too, don't think that will be a problem.

My main concern is my own personal ignorance of the NEC's standards for placement of double gang boxes in a back to back setting like this. I am probably completely wrong about this but I thought I had heard or read at one time that switch boxes that were back to back had to be at least 24 inches apart.

My local building inspectors will obviously be looking my electrical work over before I hang the drywall but I didn't want to get all this wiring in place only to find out at the rough-in inspection that it would not pass code. -- Robert
 
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Old 05-31-01, 08:23 AM
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BigRobertInKC,

What gauge wire are you going to use for the lights and switches? 12AWG? 14AWG?
 
  #5  
Old 05-31-01, 09:31 AM
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I am planning on using 12AWG for all of the switches. Four of the switches will be on a 20 AMP circuit and the other four will be on a 15 AMP circuit. None of the switches will be three-way switches. Two of the switches on the 15 AMP circuit will be dimmers and the other two switches on the 15 AMP circuit will be for duplex wall outlets. Two of the switches on the 20 AMP circuit MAY be dimmers (but probably not), one will be a "regular" single pole switch running to ceiling lights, and the last switch on the 20 AMP circuit will run to a duplex wall outlet. -- Robert
 
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Old 05-31-01, 11:10 AM
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Do you plan on having different sized conductors in the switchboxes? Both 12 awg and 14 awg in the same box? The reason I ask is because the NEC has rules on box fill (total number of conductors in a box). Thanks.
 
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Old 05-31-01, 11:19 AM
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No. Both of the switches within the same double gang box would have the same gauge wire (either 12 or 14). I have not bought my boxes yet but the double gang fiberglass boxes I am planning on using are 37.0 cubic inches. -- Robert
 
  #8  
Old 05-31-01, 12:50 PM
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You're stretching the box fill with the #12 awg wires. You'll be all right with the 14 awg wires in the boxes.
 
  #9  
Old 05-31-01, 01:53 PM
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Thinman,

Thanks for your help. Before we get too far from my original question, can I assume that you see no problem with four double gang boxes (wired correctly, under box fill limitations, etc.) spread one to each side in this square foot square I am considering?

Now, in regards to the box fill issue, I don't mean for this to be a challenge to you but I'm hoping you would be willing to educate me on this. Here's the numbers I've been using -- please point out where I'm off (if I am).

My boxes probably will have one 12 AWG wire coming in the top to bring power to the switches and two 12 AWG wires going out the bottom to take power to the loads. My understanding was that each 12 gauge cable requires 4.5 cu. in. of volume for a total of 13.5 cu. in. (3 cables x 4.5 cu. in. per cable). In addition, the grounding wires require 2.25 cu. in. bring the subtotal up to 15.75. Finally, the two switches each require 4.5 cu. in. (maybe higher depending upon the dimmer switches) for a total of 24.75 cu. in. (15.75 + 4.5 + 4.5). I'll still have some pigtail wires but, even with the pigtails added, I would still be below a cable fill violation, wouldn't I? -- Robert
 
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Old 05-31-01, 02:40 PM
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Talking

BigRobertInKC

Are you going to route an individual 12/2 w/ground power cable to each of the two boxes? Or are you going in one box and out to the second box with 12/2 w/ground cable. If you do scenario one you'll be OK. If you do scenario 2 then you may be maxed-out at the first box?
 
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Old 05-31-01, 03:19 PM
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Now I understand your concern and, frankly, I had not thought this along that far. Scenario two probably would be the likely scenario. And you're right I would actually have four 12/2 w/ground cables going in and out of the first box -- one for the incoming power, one for the outgoing power (which would then become the incoming power for the second box), and two for the loads -- so I would have an extra 4.5 cu. in. that I hadn't figured in.

What exactly is the max level for cable fill? In my 37.0 cubic inch box, am I allowed 37.0 or is it a number less than that?

If I'm going to have cable fill problems, I'll have to rework my circuit layout so that I can either get these circuits down to 15 amps so I can use 14/2 cable or to bring an individual 12/2 cable into each double gang box (scenario one in your last post).
 
  #12  
Old 05-31-01, 05:30 PM
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It sounds like you already understand the cable fill calculation very well. If your calculations say that 12-gauge wire is okay, then use it. No sense guessing -- the calculation will tell you what to do. I've put four 12-gauge cables in the same double-gang box with two switches a number of times without trouble.
 
  #13  
Old 06-01-01, 08:44 AM
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Red face

BigRobertInKC,

If you're going to use 12/2 w ground for all the circuits, then you will have a box fill problem in two of the 4 boxes.
#12 AWG requires, by NEC, 2.25 cu. in. of free space per conductor. Let's walk through this.
1. 12/2 with ground cable = 3 wires.
2. 4 cables in a box X 3 = 12 wires.
3. 12 wires X 2.25 cu. in. = 27 cu. in.
4. Count only 1 equipment grounding conductor X 2.25=2.25 cu. in. per box
5. 2 switches per box count as 4 X 2.25 cu. in. = 9 cu. in.

Add items 3, 4 & 5 and it equals at least a 38.25 cu in. box. You have 37 cu. in. boxes, right? So you're 1.25 cu. in. over.

You could route 12/3 with ground to the lights from the boxes. Route the 12/3 all way to through the first set of lights and then change to 12/2 with ground from there to the first outlet box for the second set of lights.
Using 12/3 with ground changes the total box fill to 33.75 cu. in. Make sense?
 
  #14  
Old 06-01-01, 09:01 AM
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BigRobertInKC,

Found the below statement at http://www.carlon.com.

Can Carlon Zip Boxes be installed at less than the horizontal spacing of 24" required by the model building codes?
Yes, there is a UL fire resistive wall design which permits a minimum 7" spacing on opposite sides of the wall in the same cavity space. Also, 3M offers a protective material (Putty Pad) which permits adjacent spacing of the boxes on opposite sides of the wall. Contact Carlon Technical Service for additional information
 
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Old 06-01-01, 11:23 PM
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Smile

Got it.

Thinman and John,

Thanks for all your help with this. -- Robert
 
  #16  
Old 06-02-01, 09:53 AM
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Thinman, I believe your calculation is incorrect.

A 12/2 cable only counts as TWO wires, not THREE. This changes the calculation considerably, and everything fits with plenty of room to spare.
 
  #17  
Old 06-04-01, 09:18 AM
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John,

I beg to differ about the calculation. NEC article 370-16(b)(1) Conductor Fill states, "Each conductor that originates outside the box and terminates or is spliced within the box shall be counted once,.... The egcs in the calc originate outside the box.
 
  #18  
Old 06-04-01, 10:30 AM
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Thinman,
What about 370-16(b)(5)?
Don
 
  #19  
Old 06-04-01, 10:47 AM
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Don,

I appreciate you responding to this debate. I was hoping for another point of view on this. However, as a do-it-yourselfer, I do not have the full NEC. Could you expand a little bit more on your post, please? You don't have to quote the full subsection but a high level summary would be helpful to me if you wouldn't mind.

Thanks. -- Robert
 
  #20  
Old 06-04-01, 01:24 PM
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Don,

I'm under the impression that 370-16(b)(5) is used for only "single volume allowance." So Don, you think my calculation is incorrect? Thanks.
 
  #21  
Old 06-04-01, 01:55 PM
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thinman,
In my opinion 370-16(b)(5) says that you count the 12-2 cables as 2 conductors each and then add only one allowance for all of the #12 EGCs. In this case that would give you 13 X 2.25 or 29.25 cu. in.
Don(resqcapt19)
 
  #22  
Old 06-04-01, 07:30 PM
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Thinman, I've seen many example calculations in various books, and they only count a 12/2 as two conductors. Otherwise, I think you'd be double-counting the ground wires. I recommend "Wiring Simplified" by Richter and Schwan, a green paperback sold at most Home Depot stores in the electrical aisle.
 
  #23  
Old 06-05-01, 02:15 PM
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Red face

BigRobertInKC,

John is correct. Therefore, you will have plenty of space inside the switch boxes for the #12 AWG conductors. Sorry about the misinformation.
 
  #24  
Old 06-05-01, 10:39 PM
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Thinman,

Don't worry about. I appreciate the help you did provide and your concern. You did point out to me that I would have four wires going into the first box of each pair of double gang boxes which means my cubic inches was 29.25 instead of the 24.75 I originally calculated.

By the way, the main book I've been using is part of the For Pros / By Pros series and is simply called Wiring a House by Rex Cauldwell. It states, "Add 2.25 for ALL (emphasis added by me) the grounding wires". However, it doesn't go into any details about why this is so I was willing to let you experienced folks debate this. (Still, all in all, an excellent book that I would highly recommend).

Finally, here in Kansas City, I had difficulty finding the previously referred to 3M Fire Protection Moldable Putty Pads (at least at a retailer -- some wholesalers had it but would not sell to me). I just placed the order today so I can't comment on this company's service but I found the product at http://www.whitecapdirect.com . It sells for about $5 for a 16 square inch sheet and about $6 for a 81 square inch sheet plus shipping and handling. The http://www.3M.com website had some good info on it. I'm not sure if I NEED this putty or not but I figure I would use it just for the added protection if nothing else.

Thanks also to John and Don for helping flush this out.
 
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